The detective who led the April Jones murder inquiry has been put in charge of the investigation into the deaths of two soldiers during SAS selection.

Detective Superintendent Andy John is heading the investigation into the deaths of the reservists, who collapsed last weekend in searing temperatures in the Brecon Beacons.

John won praise for building a solid case against five-year-old April's killer, Mark Bridger, despite having to run a complex and high-profile murder inquiry without a body.

No postmortem examinations have yet been carried out on Lance Corporal Craig Roberts and an unnamed colleague, who both died on Saturday, but police hope an inquest will open later this week or early next week.

The condition of a third man who also apparently succumbed to heat exhaustion remained serious on Tuesday.

Further details of Roberts, a member of the Territorial Army, were given by his family. He was working as a teaching assistant in a school in south-east London but was due to begin a new post in the office of the education secretary next month.

In a statement released via the Ministry of Defence, his family said: "We are all devastated at the loss of our beloved Craig; this has left a massive hole in all our lives.

"We wholeheartedly supported Craig in his military endeavours and it gives us some comfort, though great sadness, that he died in the pursuit of his dream. Next month Craig was due to start a new post in the office of the secretary of state for education and we were all very proud of him."

The Department for Education said Roberts, 24, had won a place on a "prestigious training scheme for high-flying graduates".

Relatives of the second man have asked that he is not named but the Guardian understands that like Roberts he was an experienced soldier trying out for the TA section of the SAS. Both are believed to have served in Afghanistan.

That both men who died were experienced and had worked in much hotter places than south Wales will increase pressure on the army and MoD to explain what went so badly wrong.

Philip Hammond, the defence secretary, promised a full service inquiry into the tragedy following the police investigation.

Det Supt John is heading a small team from Dyfed-Powys headquarters in Carmarthenshire. Health and safety executive officers are also assisting.

The inquiry has been low key so far. No appeals for witnesses to come forward have been made or details given of what may have happened.

It is not known if the team has spoken to civilian hikers on the hills, some of whom saw the soldiers in trouble. An eyewitness, Mark Nash, told Channel 4 News on Tuesday that one of the soldiers stopped to ask for water.

Nash said: "He [asked me] 'I don't suppose I could draw some water off you guys because my mate's feeling it a bit.' The other guy didn't say anything, he just looked pretty rough really.

"What I do find odd was that there didn't seem to be anybody there supervising. I would have thought they'd have people standing about at certain points to be of assistance if somebody was struggling. There could have been, but we certainly didn't see it".